Various attempts have been made to construct vaccine vectors from viruses. The use of viruses and virus proteins in eukaryotic host-vector systems has been the subject of a considerable amount of investigation and speculation. Many existing viral vector systems suffer from significant disadvantages and limitations which diminish their utility. For example, a number of eukaryotic viral vectors are either tumorigenic or oncogenic in mammalian systems, creating the potential for serious health and safety problems associated with resultant gene products and accidental infections. Further, in some eukaryotic host-viral vector systems, the gene product itself exhibits antiviral activity, thereby decreasing the yield of that protein.
In the case of simple viruses, the amount of exogenous DNA which can be packaged into a simple virus is limited. This limitation becomes a particularly acute problem when the genes used are eukaryotic. Because eukaryotic genes usually contain intervening sequences, they are too large to fit into simple viruses. In the case of complex viruses, size of exogenous DNA to be inserted is not a limiting factor. However, because they have many restriction sites, it is more difficult to insert exogenous DNA into complex viruses at specific locations.
Studies with vaccinia virus have demonstrated that poxviruses in general have several advantageous features as vaccine vectors. Poxviruses are taxonomically classified into the family Chordopoxvirinae, whose members infect vertebrate hosts, e.g., the Orthopoxvirus vaccinia. Vaccinia virus has recently been developed as a eukaryotic cloning and expression vector (Mackett, M. et al., [1985] DNA Cloning, Vol. II, ed. D. M. Glover, pp. 191-212, Oxford: IRL Press; Panicali, D. et al. [1982] Proc. Natl. Acad. Sci. USA 88:5364-5368). Numerous viral antigens have been expressed using vaccinia virus vectors (Paoletti, E. et al. [1986] Proc. Natl. Acad. Sci. USA 81:193-197; Pictinc, A. et al. [1986] BioEssays 5:248-252) including, among others, HBsAg, rabies G protein and the gp120/gp41 of human immunodeficiency virus (HIV). Regulatory sequences from the spruce budworm EPV have been used previously with vaccinia (Yuen, L. et al. [1990] Virology 175:427-433).
The advantages of poxviruses as vaccine vectors include the ability of poxvirus-based vaccines to stimulate both cell-mediated and humoral immunity, minimal cost to mass produce vaccine and the stability of the lyophilized vaccine without refrigeration, ease of administration under non-sterile conditions, and the ability to insert at least 25,000 base pairs of foreign DNA into an infectious recombinant, thereby permitting the simultaneous expression of many antigens from one recombinant.
However, although recombinant vaccinia viruses have demonstrated great potential as vaccines, vaccinia has several drawbacks that have prevented its widespread use in both human and veterinary medicine. First, vaccinia virus has a wide host range that includes man and many domestic animals. This could permit the spread of a recombinant vaccinia vaccine to other animal populations not intended for vaccination. Secondly, although the vaccinia virus recombinants are attenuated by inactivation of the thymidine kinase gene, this virus still has the ability to cause potentially serious generalized vaccinal infections in immunocompromised individuals (i.e., patients with AIDS).
Another poxvirus, swinepox virus, the only known member of the genus Suipoxvirus, is naturally restricted to swine and occurs widely throughout the world. Swinepox virus produces a mild, self-limiting disease in swine. (Kasza et al. [1960] Am. J. Vet. Res. 21:269-272; Shope [1940] Arch. Gesamte. Virustorsch 1:457-467). This virus is characterized by a genome 175 kb in size which includes a thymidine kinase (TK) gene closely resembling the TK genes of other poxviruses (Feller et al. [1991] Virol. 183:578-585).
Pseudorabies is one of the most important diseases affecting the swine industries of the USA and several countries within Europe. Losses due to disease in the USA each year run into several million dollars. This disease is characterized in its reservoir host, swine, by central nervous system disorders in suckling pigs, respiratory disease in growing pigs, and fever and inappetence in adult swine. Cattle, dogs, cats, and other species are atypical hosts of pseudorabies, but develop an invariably fatal neurological disease similar to rabies. Highly pathogenic strains of pseudorabies have been detected in the USA and later in Europe since the 1960's (Gustafson, D. P. [1986] "Pseudorabies," in Diseases of Swine, pp. 274-289, 6th Edition, eds. Leman, A. D., Straw, B., Glock, R. D., Mengeling, W. I., Penny, R. H. C., and Scholl, E., Publ. ISU press, Ames, Iowa).
The control and the eradication of pseudorabies in the USA has proven to be difficult due to the existence of a large population of feral swine in the southern states which is known to be infected with pseudorabies virus.
There remains a need for a safer and effective vector system to create vaccines directed to diseases of humans and animals, including e.g., pseudorabies infections of swine.